POPULATION AND SITE CHARACTERISTICS OF A RECENTLY DISCOVERED DISJUNCT POPULATION OF CROTON ALABAMENSIS (EUPHORBIACEAE)

19 Pages·1994·9.3 MB·English

by  Gregory H Aplet

AND
POPULATION CHARACTERISTICS
SITE
OF A RECENTLY DISCOVERED DISJUNCT
CROWN
ALABAMENS
POPULATION OF
IS
(EUPHORBIACEAE)
GREGORY
APLET
H.
The
Wilderness Society
NW
900
Seventeenth Street
DC
20006, U.S.A.
Washington,
MAURYA FALKNER
RICHARD LAVEN
D. and
B.
Department
Forest Sciences
of
Colorado State University
CO
80523, U.S.A.
Port Collins,
ROBERT SHAW^
B.
Management Military Lands
Center Ecological
for of
Colorado State University
CO
80523, U.S.A.
Fort
Collins,
Chapman,
alahamenm Smith
n A. ex a Cate^
E.
d for addition to the List of Endangered and Thre;
km
known
from any previously population. This
Una poblacion alopatnca de Croton alabamemis E. A. Smith ex Chapman, Categon'a 2 c
Especie Candidata, que esta siendo considerada para ser agregada a la Lista de Plantas e
km
Peligro de Extmcion, fue descubier'a a una distancia de 1000 de la poblacic
previamente conocida. En este tra^ ajo se describe la extension y estado de la poblacic
alopatrica. La poblacion de Croton esta situada a lo largo del fondo de un caiion aparentanc
poder sobrevivir crecer en buenas condiciones. La ocurrencia de Croton no parece ten(
y
INTRODUCTION
Chapman
Croton alabamemis E, A. Smith ex (Euphorbiaceae) has been de-
m
Thomas
scribed as "one of the rarest shrubs the United States" (Farmer and
m
1969). a short-lived (<20 multi-stemmed, monoecious shrub <3
It
is
y),
tall found primarily on limestone and shale outcrops along the Warrior and
Cahaba Rivers, Tuscaloosa and Bibb Alabama. was
counties, collected
It
once in 1899 in Tullahoma, Coffee County, Tennessee, but has not been
from
reported there subsequently. Croton alabamemis has long been thought
grow
to only in isolated populations within these two neighboring counties.
Due
to its restricted range, it is being considered for addition to the List of
Endangered and Threatened Plants under the Endangered Species Act of
amended.
1973, as It currently designated as a Category 2 Candidate,
is
which means some
that "there evidence but
is for vulnerability, there are
. . .
enough
not data support (USDI
to listing proposals time"
at this 1991).
km
In early 1990, Croton alahamenm was discovered over 1000 from any
known
previously population on Army's Hood, The
the U.S. Fort
Texas.
population was discovered by John Cornelius, a wildlife biologist with the
Hood
Management
Fort Resource Department, during an excursion view
to
Owl
bird habitat in the Creek Mountains. Cornelius showed the population
weeks Beardmore
several later to Carol of the U.S. Fish and Wildlife Service
Rex Wahl
and
of Texas Parks and Wildlife. Ginzbarg (1992) mistakenly
credited the discovery to Beardmore and Wahl. Not long afterwards, a
second and a third population were discovered in Travis County, to the
The
south. species identity was determined by Steve Ginzbarg, graduate
a
student in botany at the University of Texas, Austin. Ginzbarg (1992)
subsequently determined
that the Texas plants
are sufficiently distinct to
He
merit consequently them
varietal status. described Croton alabamemis
as
Chapman
Smith
E. A. ex var. texemis Ginzbarg (Ginzbarg 1992). Key
characteristics of the Texas variety are represented in Figures and
2.
1
we
In June 1991, undertook to describe the extent and status of the
Hood
population Fort Department
at as part of the of Defense's Legacy
Resource Management Program. The Legacy Program was
established to
Our
military lands. study aimed at gathering the requisite information to
is
We
effect proper stewardship of this species at Ft. Hood. therefore restricted
our analysis to only one of the three Texas populations the Hood
Ft.
(i.e.
We
recommend
population). that similar studies be undertaken both
in
FIG. Croton alahamensts var. texei
1.
rimary and secondary branches
(flow(
lowing developed
fully leaves.

Alabama
Texas and to assess the status of populatii
all
important
scientifically species.
Owl
This study was conducted in several small watersheds of the Creek
Mountains, and Country
Bell Coryell counties, in the Hill of central Texas
Owl ~300
(31°N, 97°W). The Creek Mountains m) com-
(elevation are
posed of Mesozoic limestone overlain by clayey and loamy (McCaleb
soils
m
-65
The summit
1985). formation to plateau above the west-to-
rises its
km
Owl
east flowing Creek in than 2 Intermittent tributary streams run
less
.
northward to the creeksuch that virtually canyon walls have east- and
all
west-facing
aspects.
The
climate hot in the summer, and the winters are generally mild, with
is
The summer
an occasional cold surge. average daily temperature during the
maximum
28°C, with an average daily of 36°C. Winter daily tempera-
is
tures average 9°C, with an average low of 3°C. Rainfall distributed uni-
is
formly throughout the year, with a slight peak in the spring and an average
mm
Owl
annual total of 825 (McCaleb 1985). Creek flows to some extent
throughout the year, but the tributary streams are dry most of the year.
The Appendix complete
vegetation of the area (see for plant species
1 list)
primarily Ashe juniper Buchh.) woodland of the appropriate
(Juniperus ashei
is
meet two
character to the habitat needs of rare birds, the black-capped vireo
{Vireo atrkapilla) and the golden-cheeked warbler {Dendroka chrysoparia).
The summit
vireo primarily inhabits the shrubby balds of the plateau, but
the warbler requires the bark of relatively old junipers for nesting materials
and mature hardwoods feeding Cornelius comm.).
for pers.
(J.
Prior to our study, Croton alabamemis var. texensis had been identified from
We
Owl
two
of the tributary canyons of Creek. searched additional
five
canyons and found only three more plants in the canyon between the original
Owl
two and several plants along the stretch of Creek between the two
two
tributaries; populations Croton appear to be largely restricted to the
oi^
Our
two
original canyons. study involved phases: a description of the
structure and habitat of the two Croton populations (canyons and and an
1 3)
why
attempt to discern Croton but absent from the canyon between
is all
(canyon Subsequent to the completion of this study, another population
2).
km
consisting of 35 individuals was located in a canyon three east of the
study This population not considered our
area. in analysis.
is
The mapped
bed of each of the three canyons was using a tape and a hand-
The
eld compass. slope of the creek was measured using hand-held
a
iinometer. Three high density stands of C. alabamensis var. texensis were
sampling along the creekbeds Canyons and
identified for intensive in 1 3.
At each sample location, two transects were established perpendicular to the
creekbed to facilitate sampling of both east- and west-facing aspects. In
Canyon which were
lacked Croton, similar pairs of transects established at
2,
up canyon comparable Canyons and
distances the to those identified in
1 3.
Slope topography, overstory and understory cover, Croton seedling den-
Croton adult population structure, and soil depth were determined
sity,
along each At each tape was stretched upslope beyond the
transect. a
site,
m
some was 60 from
extent of the Croton population. In cases, this as far as
m
Canyon 30-40
the creekbed, but was generally Transects in 2 were
less.
long. Slope breaks and important topographic features were noted along the
map
and was prepared each depths
slope, a cross-section for transect. Soil
m
were measured 10 and important topographic by
at intervals at features
aim
probing with
the rod.
soil
m m
Along the cover of overstory (>2 and understory (<2 but
tape, tall)
>10 cm vegetation was determined using the line-intercept method
tall)
and EUenberg The
(iVlueller-Dombois 1974). line also served as the center
m
cm
wide which <30 were
of a 2 Croton seedling belt in individuals tall
tallied.
cm
>30
Croton were sampled using a modified nearest-neighbor
tall
method adapted from those described by Muelier-Dombois and EUenberg
80°
(1974). Starting at the beginning of the line (middle of the creekbed), a
1
cm
>30 The
arc was searched upslope for an individual of Croton
tall.
distance and azimuth to the nearest plant were recorded as were the height
of the tallest shoot and the diameter of each live shoot (to the nearest 0.5 cm).
From
that plant, the process was repeated until no plants could be found
m
within 10 of the last plant (Fig In only one case, the process was
3).
moved when >10
suspended and back to the tape the search led to a plant
m
m
from the tape. Thus, plants were sampled within 10 of the tape.
all
Consistent with other nearest-neighbor methods, the distance between
plants was assumed to be related to the share of the total area allotted to an indi-
From
vidual plant. these data, population density could be calculated as the
number sum
total of plants sampled divided by the of areas allotted to indi-
vidual plants. Additionally, using a simple trigonometric conversion, the den-
represented by individual plants could be plotted against distance from
sity
the creek to assess changes in density related to environmental variables.
Sizes of individual, multi-stemmed Croton plants were described using the
of equivalent diameter Equivalent diameter the diameter
statistic (Dgq).
is
sum
stem same
of a single that possesses the cross-sectional area as the of all
the stems borne by the plant. For example, as the total cross-sectional area
cm
X
of four shoots, each 2 in diameter (4 3.14 cm^) equal to that of one
is
cm
shoot, 4 in diameter (12.56 cm^), the Deq for the four shoots 4 cm.
is
Ant
ecology
The
population two
structures of Croton alabamensis var. texensts in the
canyons are presented in Figure 4. In both canyons, Croton was well
represented in the smaller diameter classes, but also occurred as large mature
plants. This decreasing monotonic population structure was found
in
all
which was
transects in Croton relatively dense. Therefore, both canyons
appeared to support healthy populations of adults, juveniles, and new
Croton density was highly variable within Canyons and Each canyon
1 3.
contained places in which scarcely an individual was found. In other places,
Croton formed dense thickets in which dominated to the near exclusion of
it
The
other understory species. densest stands occurred in Canyon where
1
Croton density (adults and seedlings) in two transects exceeded 100 plants/
100 m^, but three transects in Canyon exceeded 50 plants/100 m^ (Table
3
km
we number
1). Overall, estimate the of plants occurring in the 1.5 of
~
creekbed in Canyons and to be 20,000 individuals.
3
1
No
In general, Croton was found only in the canyon bottoms. individuals
found from
the ndoftl
1
s c
;
m
>40
nly four contained plants from the creek. Croton density fluctuated,
ut Croton did not decline proportionally with distance from the creek.
Canopy
gaps along the line intercept showed no correlation with Croton
ccurrence. Croton were found in openings and in the deepest shade. The can-
Owl
ons of the Creek Mountains have been subjected harvesting of
to juni-
may
and some
er trees for fence posts, of the gaps have been anthropogenic.
Canyon
1
12 14 >15
10
4 6 8 10 12 14 >15
Deq Class
(nnm)
VEGETATION COVE
2
^
'^^^ 1
1
11
42
I
E
'4I
84 49 ?o'o 600 2'i
2
3
96 54 60.0 E 350 55.8
9.7
3 3
W
99 47 30.0 325 23.4 28.6
3 5
In some cases, the loggers incidentally cut adult Croton along roads and skid
but the plants sprouted and were growing well.
trails,
Topographic features also showed no direct correlation with Croton
density. Croton occurred on bank slopes, terraces, and on toeslopes of each
much
canyon. Soil depth, however, explained of the variability in Croton
A
distribution. comparison of depth with the presence of Croton along
soil
with deep
the transect indicated a significant association of adult plants soil
Our
(p<0.05; Kruskal-Wallis inability to measure depths in excess of
test).
m
mean
means that the measured depth in the presence of Croton (0.80
soil
1
m; n = 64) was probably considerably less than the true mean. In contrast, soil
m
measurements in the absence of Croton averaged only 0.42 (n = 33) in
Canyons and
1 3.
Comparison with composition
of Croton density overstory species sug-
gests an association with mesic would be expected from the observed
sites, as
restriction of Croton to the canyon bottoms. Figure shows the relationship
5
combined
of adult Croton density to the cover of Fraxinus texensis (Gray) Sarg.
and Quercus muhlenbergii Engelm. and to the combined cover ofJuniperusashei
m
and Quercus texana Buckl. within 40 of the creek. These four species
woody
dominated the overstory stratum in these canyons; only the vine Vitis
Of
mustangensis Buckl. contributed comparable cover over the study the
area.
—
0.2 0.4 0.6 0.8 1.0
^
and Cover
Juniperus texana
0.
0.0 0.2 0.3 0.4 0.5
0.1
Fraxinus and muhlenbergii Cover
Q.